The present invention is directed to a method of rehabilitating and stabilizing salt-contaminated reinforced concrete structures, and a novel composition used in the method.
It is well known that reinforced concrete structures, such as bridges, parking garages, and the like are highly susceptible to corrosion and degradation from commonly applied chloride deicing salts. It is believed that a large percentage of all bridge decks in the United States, and in other countries which have cold climates, are seriously deteriorated because of corrosion of the reinforcing steel which is part of their structure. This corrosion is usually caused by chloride ions that have penetrated the concrete as a result of repeated application of deicing salts. The mechanism by which this corrosion and degradation occurs may be explained as follows:
Under highly alkaline conditions the reinforcing steel has formed upon it an oxidized film of .gamma.-Fe.sub.2 O.sub.3, which inhibits the corrosion of the steel material; the steel is thus said to have been "passivated". However, when chloride ions are introduced into the cementitious matrix, the passivated film is broken. (In concrete parking structures, for example, automobiles carry salt-contaminated ice and snow to the structure and, while parked, allow the contaminated ice and snow to melt, concentrating the salt on the concrete structure.) The breakage of the film generally occurs locally to expose the steel material. As a result, the smaller exposed part of the steel surface acts as an anode, and the larger portion, still covered by the passivated film, acts as a cathode to cause a large potential difference between them and, therefore, only the anode (smaller part) is corroded. As a result, so-called "pitting corrosion" takes place in spots on the surface of the steel.
The effective sectional area of the steel material is rapidly reduced by the pitting corrosion and such corrosion is therefore dangerous, even when the number of the spots is small. When the number of pits increases, they interconnect to finally spread all over the steel surface. In the initial stages of corrosion, ferrous hydroxide (Fe(OH).sub.2) is formed. This compound is unstable and immediately oxidized into iron oxides such as .alpha.-FeOOH and Fe.sub.3 O.sub.4, which are the main components of rust. In the course of the rust formation, the steel expands.
As such, a high expansion pressure is applied to the concrete matrix, forming cracks along the embedded rods. As cracks further develop in the concrete surface, additional chloride solution gets to the steel, accelerating corrosion and spalling of the concrete structure. If corrosion and spalling are allowed to continue, the metal reinforcements, as well as the surrounding concrete, deteriorate to a point which requires substantial removal and replacement of the entire structure. This is a difficult and costly endeavor.
Several methods have been suggested to ameliorate the condition of concrete structures which have undergone or are susceptible to corrosion deterioration. For example, low-permeability overlays have been applied to deteriorated structures. In such instances, spot repairs for severely deteriorated concrete is first accomplished. However, large areas of chloride-contaminated concrete remain in place and, although substantially slowed, corrosion and deterioration continue to occur. Thus, this method alone does not address the need for a long-term rehabilitation procedure.
A rehabilitation technique which is often used involves scarifying the top portion of, e.g., a bridge deck prior to application of a new overlay. Scarification to within 1.0 cm of the embedded reinforcement metal elements removes a major amount of contaminated concrete, permits impregnation of corrosion-inhibiting agents to the concrete around the steel reinforcements and then application of a new concrete overlay. A preferred mode requires complete removal of the concrete surrounding the steel reinforcements prior to applying a new overlay.
However, the corrosion-inhibiting compositions that have been used until now for impregnating concrete with corrosion-inhibiting agents have been somewhat limited in their ability to deliver the agents deeply into the concrete matrix. Another problem relates to even application of these compositions; when treating a concrete surface, e.g., by spraying, it is difficult for the applicator to determine whether the composition has been evenly applied.
As such, it is an object of the invention to provide improved corrosion-inhibiting compositions, and methods of applying them, that address these problems.